It is known from practice, and for instance from EP 0 667 415 A1, that certain self propelled building machines, e.g., road finishers or chargers, have to be supplied with material during their operation. This process, which is called charging, is carried out during the driving operation, for instance by a truck driving backwards and approaching the road finisher or charger, which moves forwards at the same time, and tipping the material from its load area into the material hopper of the road finisher or charger. While the charging takes place the truck is pushed forward by the road finisher or charger. This means that some coordinated driving maneuvers are necessary to enable the charging. As a whole, this charging process therefore requires a high degree of coordination between the driver of the road finisher or charger and the truck driver. For this reason EP 0 667 415 A1 proposes a means for monitoring and displaying parameters regarding the charging with road-laying material, which is mounted on the road finisher or charger so as to display certain commands or signals regarding the operation to the truck driver. The disadvantage of the prior signaling devices is that the attachment thereof to the road finisher or charger is complicated. Frequently these signaling devices are mounted at places that restrict the viewing area of the driver of the road finisher or charger and, thus, have a negative influence on the driving safety.